The interleukin (IL)-13 is a pleiotropic T helper cell subclass 2 (Th2) cytokine. Like IL4, IL13 belongs to the family of type I cytokines sharing the tertiary structure defined by a 4α-helical hydrophobic bundle core. IL13 has approximately 30% amino acid sequence homology with IL4 and shares many of the properties of IL4 (Wynn, Ann. Rev. Immunol., 21: 425 (2003)). The functional similarity of IL4 and IL13 is attributed to the fact that IL13 can bind IL4 receptor alpha chain (IL4R-α) subsequent to its binding to IL13 receptor alpha chain-1 (IL13Rα1) (Hershey, J. Allergy Clin. Immunol., 111: 677 (2003)). IL4Rα is activated by IL4 and IL13 resulting in Jak1-dependent STAT6 phosphorylation. Both IL4 and IL13 promote B-cell proliferation and induce class switching to IgG4 and IgE in combination with CD40/CD40L costimulation (Punnonen et al., Proc. Natl. Acad. Sci. USA, 90: 3730 (1993), Oettgen et al., J. Allergy Clin. Immunol., 107: 429 (2001)).
However, unlike IL4, IL13 is not involved in the differentiation of naïve T cells into Th2 cells (Zurawski et al., Immunol. Today, 15: 19 (1994)). IL13 up-regulates FcεRI and thus helps in IgE priming of mast cells (de Vries, Allergy Clin. Immunol. 102: 165 (1998). In monocytes/macrophages, IL13 up-regulates expression of CD23 and MHC class I and class II antigens, down-regulate the expression of Fcγ and CD14, and inhibit antibody-dependent cytotoxicity (de Waal Malefyt et al., J. Immunol., 151: 6370 (1993), Chomarat et al., Int. Rev. Immunol., 17: 1 (1998)). IL13, but not IL4, promotes eosinophil survival, activation, and recruitment (Horie et al., Intern. Med., 36: 179 (1997), Luttmann et al., J. Immunol. 157:1678 (1996), Pope et al., J. Allergy Clin. Immunol., 108: 594 (2001). IL13 also manifests important functions on nonhematopoietic cells, such as smooth muscle cells, epithelial cells, endothelial cells and fibroblast cells. IL13 enhances proliferation and cholinergic-induced contractions of smooth muscles (Wills-Karp, J. Allergy Clin. Immunol., 107: 9 (2001). In epithelial cells IL13 is a potent inducer of chemokine production (Li et al., J. Immunol., 162: 2477 (1999), alters mucociliary differentiation (Laoukili et al., J. Clin. Invest., 108: 1817 (2001), decreases ciliary beat frequency of ciliated epithelial cells (Laoukili et al., J. Clin. Invest., 108: 1817 (2001), and results in goblet cell metaplasia (Zhu et al., J. Clin. Invest., 103: 779 (1999), Grunig et al., Science, 282: 2261 (1998)). In endothelial cells IL13 is a potent inducer of vascular cell adhesion molecule 1 (VCAM-1) which is important for recruitment of eosinophils (Bochner et al., J. Immunol., 154: 799 (1995)). In human dermal fibroblasts IL13 induces type 1 collagen synthesis in human dermal fibroblasts (Roux et al., J. Invest. Dermatol., 103: 444 (1994)).
Although IL13 and IL4 share certain functional similarities, studies in animal models of disease and gene-knockout mice demonstrated that IL13 possesses unique effector functions distinct from IL4 and provides compelling evidence that IL13, independent of other Th2 cytokines, is necessary and sufficient to induce all features of allergic asthma (Wills-Karp et al. Science, 282: 2258 (1998), Walter et al. J. Immunol. 167: 4668 (2001)). IL13 may play a more significant role than other Th2 cytokines in effector functions associated with the symptoms of asthma (Corry, Curr. Opin. Immunol., 11: 610 (1999)). This contention is supported in human disease by a strong association between IL13 levels and genetic polymorphisms in the IL13 gene and disease correlates (Wills-Karp. et al. Respir. Res. 1: 19 (2000); Vercelli et al., Curr. Opin. Allergy Clin. Immunol., 2: 389 (2002); He et al., Genes Immunol., 4: 385 (2003), Arima et al., J. Allergy Clin. Immunol., 109: 980 (2003), Liu et al., J. Clin. Allergy Immunol., 112: 382 (2003)). Emerging data suggest that IL13 induces features of the allergic response via its actions on mucosal epithelium and smooth muscle cells, rather than through the traditional pathways involving eosinophils and IgE-mediated events (Wills-Karp et al., Sci., 282: 2258 (1998)).
Asthma is described as a chronic pulmonary disease that involves airway inflammation, hyperresponsiveness and obstruction. Physiologically, airway hyperresponsiveness is documented by decreased bronchial airflow after bronchoprovocation with methacholine or histamine. Other triggers that provoke airway obstruction include cold air, exercise, viral upper respiratory infection, cigarette smoke, and respiratory allergens. Bronchial provocation with allergen induces a prompt early phase immunoglobulin E (IgE)-mediated decrease in bronchial airflow followed in many patients by a late-phase IgE-mediated reaction with a decrease in bronchial airflow for 4-8 hours. The early response is caused by acute release of inflammatory substances, such as histamine, PGD2, leukotriene, tryptase and platelet activating factor (PAF), whereas the late response is caused by de novo synthesized pro-inflammatory cytokines (e.g. TNFα, IL4, IL13) and chemokines (e.g. MCP-1 and MIP-1α) (Busse et al. In: Allergy: Principles and Practice, Ed. Middleston, 1173 (1998)). In chronic asthmatic patients, persistent pulmonary symptoms are mediated by the heightened response of Th2 cells. Th2 cytokines are believed to play a vital role in the disease (Larche et al., J. Allergy Clin. Immunol., 111: 450 (2003)), in particular, IL13 and IL4 produced by Th2 cells with NK phenotype (NKT) in the airway as indicated in a model of asthma in rodents (Akbari et al., Nature Med., 9: 582 (2003)). The gross pathology of asthmatic airways displays lung hyperinflation, smooth muscle hypertrophy, lamina reticularis thickening, mucosal edema, epithelial cell sloughing, cilia cell disruption, and mucus gland hypersecretion. Microscopically, asthma is characterized by the presence of increased numbers of eosinophils, neutrophils, lymphocytes, and plasma cells in the bronchial tissues, bronchial secretions, and mucus. Initially, there is recruitment of leukocytes from the bloodstream to the airway by activated CD4+ T-lymphocytes. The activated T-lymphocytes also direct the release of inflammatory mediators from eosinophils, mast cells, and lymphocytes. In addition, the Th2 cells produce IL4, IL5, IL9 and IL13. IL4, in conjunction with IL13, signals the switch from IgM to IgE antibodies.
Cross-linking of membrane-bound IgE molecules by allergen causes mast cells to degranulate, releasing histamine, leukotrienes, and other mediators that perpetuate the airway inflammation. IL5 activates the recruitment and activation of eosinophils. The activated mast cells and eosinophils also generate their cytokines that help to perpetuate the inflammation. These repeated cycles of inflammation in the lungs with injury to the pulmonary tissues followed by repair may produce long-term structural changes (“remodeling”) of the airways.
Moderate asthma is currently treated with a daily inhaled anti-inflammatory-corticosteroid or mast cell inhibitor such as cromolyn sodium or nedocromil plus an inhaled beta2-agonist as needed (3-4 times per day) to relieve breakthrough symptoms or allergen- or exercise-induced asthma. Cromolyn sodium and nedocromil block bronchospasm and inflammation, but are usually effective only for asthma that is associated with allergens or exercise and typically, only for juvenile asthmatics. Inhaled corticosteroids improve inflammation, airways hyperreactivity, and obstruction, and reduce the number of acute exacerbations. However, it takes at least a month before effects are apparent and up to a year for marked improvement to occur. The most frequent side effects are hoarseness and oral fungal infection, i.e., candidiasis. More serious side effects have been reported, e.g., partial adrenal suppression, growth inhibition, and reduced bone formation, but only with the use of higher doses. Beclomethasone, triamcinolone, and flunisolide probably have a similar potency; whereas budesonide and fluticasone are more potent and reportedly have fewer systemic side effects.
Even patients with mild disease show airway inflammation, including infiltration of the mucosa and epithelium with activated T cells, mast cells, and eosinophils. T cells and mast cells release cytokines that promote eosinophil growth and maturation and the production of IgE antibodies, and these, in turn, increase microvascular permeability, disrupt the epithelium, and stimulate neural reflexes and mucus-secreting glands. The result is airways hyperreactivity, bronchoconstriction, and hypersecretion, manifested by wheezing, coughing, and dyspnea.
Traditionally, asthma has been treated with oral and inhaled bronchodilators. These agents help the symptoms of asthma, but do nothing for the underlying inflammation. Recognition during the last 10 years of the importance of inflammation in the etiology of asthma has led to the increased use of corticosteroids, but many patients continue to suffer from uncontrolled asthma.
Because of the importance of treating inflammatory diseases in humans, particularly asthma, new bioactive compounds having fewer side effects are continually being sought. The development of potent and specific inhibitors of IL13, which remain active when administered long term to asthmatic airways, offers a novel approach to the treatment of asthma, as well as in other IL13- and IgE-mediated diseases.